Method for fabricating an optical fiber, preform for fabricating an optical fiber, optical fiber and apparatus

ABSTRACT

The method for fabricating an optical fiber comprises the steps of inserting a primary optical fiber preform ( 11 ) having a first primary axis (x 1 ) and an outer surface ( 111 ) into an overcladding tube ( 12 ) having a second primary axis (x 2 ) and an inner surface ( 120 ), so that said outer surface and inner surface define an interior space ( 15 ); holding the primary preform ( 11 ) in a centrally inserted position within the overcladding tube ( 12 ) with said first and second primary axes (x 1 , x 2 ) in substantial alignment with each other; supplying overcladding grain ( 13 ) into the interior space ( 15 ) that is limited at the lower end of the overcladding tube ( 12 ) by means of a closure ( 125 ); generating a condition of reduced pressure within the interior space ( 15 ) that is limited at the upper end of the overcladding tube ( 12 ) by means of an adjoiner ( 3 ), which holds the primary optical fiber preform ( 11 ) and the overcladding tube ( 12 ) in position; and heating the unprocessed secondary preform ( 1 ), that consists of the primary preform ( 11 ), the overcladding tube ( 12 ) and the overcladding grain ( 13 ), at its lower end to a softened state and simultaneously or subsequently drawing an optical fiber therefrom.

The present invention relates to a method and an apparatus forfabricating an optical fiber and to a preform used for fabricating anoptical fiber as well as to optical fiber fabricated according to saidmethod.

Fabrication of optical fibers, such as the fibers currently used inultra high speed data communication networks, is described in [1], MoolC. Gupta, Handbook of PHOTONICS, CRC Press, 1997 Boca Raton, chapter10.7, pages 445-449. Main process steps of optical fiber fabrication arefabricating a glass blank (below called preform), drawing the fiber fromthe preform and coating the fiber with a material that protects thefiber from handling and from environmental influences.

According to [1], there are basically three methods to form the preform.The modified chemical vapor deposition process (MCVD), the outside vapordeposition process (OVD) and the vapor axial deposition process (VAD).

In the drawing process, the preform is fed from above into the drawingportion of the furnace while being drawn from the bottom using tractors.The fiber is then wound onto a drum while being monitored for tensilestrength. The temperature during draw is on the border of 2000° C. Afterexiting the furnace the fiber is coated with a UV-curable coating beforewinding on the drum.

As described in [2], U.S. Pat. No. 6,519,974B1, the MCVD method hascertain advantages over the other methods. In the MCVD process,successive layer of SiO2 and dopants, which include Germanium,phosphorous and fluorine, are deposited on the inside of a fused silicatube by mixing the chloride vapors and oxygen at a temperature in theorder of 1800° C. In the layer deposition process the cladding layersare laid down first, and then the layers that will form the core aredeposited. After deposition of the layers, the internally layered quartztube is heated in the presence of Cl2 and He so as to form a compactquartz rod.

As further stated in [2], the MCVD method, used by itself, has theinherent limitation that it is not suitable to make preforms of morethan 25 mm in diameter. In order to overcome this limitation, MCVD isoften practiced with a so-called overcladding method, which allowsfabrication of relatively large preforms and thus improves productivityfor the fiber fabrication process. Conventional overcladding involves,in general terms, placing a rod preform inside a tube made of a suitableovercladding material, fusing the rod and tube together to form asecondary preform, and drawing from the secondary preform an opticalfiber comprising a core enclosed within a cladding layer. Thus, a highproductivity implementation of the MCVD method requires three essentialsteps: preparing a primary optical fiber preform by internal deposition,overcladding the primary optical fiber preform to obtain a secondaryoptical fiber preform, and finally drawing an optical fiber from thesecondary optical fiber preform.

In [2], it has been found that carrying out these three steps separatelyrequires

-   a) substantial amounts of time and consequently has a negative    effect on productivity;-   b) a large amount of oxygen or hydrogen for the step of overcladding    the primary optical fiber preform; and-   c) application of a relatively large amount of heat for the    overcladding step, if the primary optical fiber preform is    relatively large.

To overcome these disadvantages a combination of the overcladding anddrawing steps was proposed e.g. in [3], U.S. Pat. No. 2,980,957. Themethod disclosed in [3] comprises the steps of creating between a corerod and a thereto concentrically arranged overcladding tube a highvacuum prior to the drawing stage and further a controlled low vacuum inorder to controllably counteract the drawing forces and cause thetubular member to progressively collapse into the space between the corerod and the overcladding tube. One problem in combining the fusing anddrawing stages has been to control the application of vacuum withsufficient precision that the finished optical fiber has sufficientstrength and optical quality for modern communications applications.

Another aspect addressed in [2] is the proper alignment of the core rodand the overcladding tube. A method has been disclosed in [4], U.S. Pat.No. 4,820,322, that allows the fabrication of a strong fiber withconcentric core and cladding, that uses a vacuum to promote collapse ofthe overcladding tube, and that can be practiced either in a separatemanufacturing phase or in a continuous process combined with drawing ofthe fiber. As stated in [2] the approach disclosed in [4] has a limit onthe gap between the rod and the overcladding tube; the tube insidediameter cannot exceed the rod diameter by more than a certain amount.Furthermore, the embodiment combining collapsing the tube and drawingthe fiber does not use an affirmative means to center the rod in thetube, relying instead for concentricity on inherent self centeringforces thought to be present as the fiber is drawn from the tip of therod-and-tube preform.

To improve the techniques described above a method has been proposed in[2] that allows drawing of an optical fiber from a rod-and-tube preformwhile simultaneously fusing the rod and the overcladding tube. Thisrod-in-tube approach employs a low intensity vacuum source that permitsfine adjustment of the differential pressure. It also provides forcontrolled alignment of the core rod and the overcladding tube to ensurethat the desired circumferential uniformity of the cladding layer in thedrawn fiber is achieved. The low intensity vacuum is achieved byintroducing a flow of gas into an adjoiner that holds a primary opticalfiber preform having a first primary axis and an outer surface and anovercladding tube having a second primary axis and an inner surfacedefining an interior space, coaxially aligned together as a secondarypreform assembly. The flow of gas through a channel in the adjoinergenerates a condition of reduced pressure in accordance with Bernoulli'stheorem, and therefore partially evacuates the space between theovercladding tube and the primary optical fiber preform. The flow ratethrough channel will determine the extent to which the gas pressure inthe space is reduced.

According to [2], the main concern with the realization of rod in-tubeprocesses is focused on alignment procedures and the application of aprecisely controlled vacuum. However, besides these known main concerns,costs for the production of high quality optical fiber from arod-and-tube preform are a continuous concern.

It would therefore be desirable to provide an improved method and anapparatus that allow fabricating high quality optical fiber from arod-and-tube preform.

It would be desirable in particular to provide a method that allowsfabricating high quality optical fiber from a rod-and-tube preform atsignificantly reduced cost.

Still further it would be desirable to provide a method that allows areduction of the requirement of precision in alignment of rod and tubeof the rod-and-tube preform as well as a reduction of the requirement ofprecision for controlling the vacuum for sequential or simultaneouspreform fusing and fiber drawing.

It would further be desirable to create a rod-and-tube preform, that canbe used with the inventive method, as well as high quality optical fiberdrawn from said rod-and-tube preform.

It would further be desirable to create a rod-and-tube preform thatallows modification of the properties of the optical fiber drawn fromsaid rod-and-tube preform with reduced effort.

SUMMARY OF THE INVENTION

The above and other objects of the present invention are achieved by amethod according to claim 1, a secondary preform according to claim 6,an optical fiber according to claim 9 and an apparatus according toclaim 10.

The method for fabricating an optical fiber comprises the steps of:

inserting a primary optical fiber preform having a first primary axisand an outer surface into an overcladding tube having a second primaryaxis and an inner surface, so that said outer surface and inner surfacedefine an interior space;

holding the primary preform in a centrally inserted position within theovercladding tube with said first and second primary axes in substantialalignment with each other;

supplying overcladding grain into the interior space that is limited atthe lower end of the overcladding tube by means of a closure;

generating a condition of reduced pressure within the interior spacethat is limited at the upper end of the overcladding tube by means of anadjoiner, which holds the primary optical fiber preform and theovercladding tube in position; and

heating, by means of a furnace preferably in the range of 2100° C. to2250° C., the unprocessed secondary preform, that consists of theprimary preform, the overcladding tube and the overcladding grain, atits lower end to a softened state and simultaneously drawing an opticalfiber therefrom orheating, by means of a furnace, the unprocessed secondary preform, thatconsists of the primary preform, the overcladding tube and theovercladding grain, substantially over its entire length in order toobtain a processed secondary preform, from which an optical fiber isdrawn in a subsequent process stage.

Due to the thermal energy provided by the furnace and due to theestablished difference of pressures that are present in and outside thesecondary preform, the overcladding tube will collapse and press themolten overcladding grain onto the primary preform.

The overcladding material of the overcladding tube and the overcladdinggrain will form a practically homogeneous layer that adjoins the primarypreform in the same manner as the thick overcladding tube does, when itcollapses, in conventional rod-and-tube applications, as described forexample in [2].

Fusing of the secondary preform and fiber drawing can be performedsimultaneously as with the method described in [2]. However theunprocessed secondary preform can also be processed in a preliminaryprocess stage in order to obtain a processed secondary preform fromwhich an optical fiber can be drawn in a subsequent process stage at thepresent or another process site.

The present invention however yields numerous advantages over the priorart mentioned above:

The known method of producing a secondary preform by sleeving anovercladding tube with thick walls over a primary preform is abandoned.Instead an overcladding tube with thin walls is used and the interiorspace between the primary preform and the inner surface of theovercladding tube is filled with silica grain. Consequently the effortand costs for the production of the overcladding preform with thickwalls are avoided. Instead of a costly silica tube with thick walls,silica grain can be used.

Due to the mobility of the overcladding grain, the interior space or gapbetween the outer surface of the primary preform and the inner surfaceof the overcladding tube is evenly filled with the silica grain, so thata misalignment between the primary preform and the overcladding tube.Besides the elimination of alignment problems the control of thepressure reduction is less critical, since the overcladding tube is notcollapsing uncontrolled into a free gap but pressing steadily onto thesupporting grain.

The inner diameter of the thin walled overcladding tube is preferablyselected at least 1.5 times larger than the outer diameter of theprimary preform and more than 10 times larger than its wall diameter.However, in practice any dimensions can be realized, that are supportedby the mechanical strength of the related elements.

Further the overcladding tube is preferably supplied with a conicallyformed closure at its lower end, so that the walls of the overcladdingtube and the primary preform meet at their lower end and silica graincan be filled into the interior space. Since the primary preform in apreferred embodiment also comprises a conical form at its lower end, thealignment procedures are significantly facilitated.

The overcladding grain, which consists of particles with a smalldiameter, e.g. a powder, is inserted into the interior space before theadjoiner is mounted or after the adjoiner is mounted, through a channelprovided therein.

The overcladding grain may be a pure or doped synthetic silica powderthat may be selected according to the desired properties of thefabricated fiber. A method of manufacturing a silica powder using asolgel technique is described in [6], U.S. Pat. No. 6,047,568. Furthersolgel techniques, for achieving higher drawing forces and reducingbreakage risks during the drawing process are described in [7], U.S.Pat. No. 6,334,338. Hence the inventive method also provides a highflexibility that allows meeting the customer's demands within shortnotice.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the objects and advantages of the present invention have beenstated, others will appear when the following description is consideredtogether with the accompanying drawings, in which:

FIG. 1 shows a primary preform 11 having a first primary axis x1;

FIG. 2 shows a thin walled silica tube 12, having a first primary axisx2, with a conical closure 125 at its lower end that, according to theinventive method, is used as overcladding tube 12;

FIG. 3 shows the primary preform 11 held in a centrally insertedposition within the overcladding tube 12 with said first and secondprimary axes x1, x2 in substantial alignment with each other;

FIG. 4 shows an unprocessed secondary preform 1 with the primary preform11 and the overcladding tube 12 of FIG. 3 with an interior space 15,that is defined by the outer surface 111 of the primary preform 11 andinner surface 120 of the overcladding tube 12, filled with overcladdinggrain 13;

FIG. 5 shows the secondary preform 1 of FIG. 4 with an adjoiner 3partially inserted into the overcladding tube 12, holding the primarypreform 11 in centralized position and closing and sealing the interiorspace 1 on its upper side;

FIG. 6 shows a secondary preform 1 with an adjoiner 3 that allowsinsertion of overcladding grain 13 through a channel 38;

FIG. 7 shows the upper end of secondary preform 1 of FIG. 5 in detail;

FIG. 8 shows the adjoiner 3 used for the secondary preform 1 of FIG. 4;

FIG. 9 shows a sectional view of the adjoiner 3 of FIG. 6, with thechannel 38 provided for the insertion of overcladding grain 13; and

FIG. 10 shows an apparatus used for drawing an optical fiber from thesecondary preform 1 of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a primary preform 11 having a first primary axis x1, anouter diameter d1 and an outer surface 111. Manufacturing of such apreform has been described above.

FIG. 2 shows a thin walled silica tube 12, having a first primary axisx2, an inner diameter d2, a wall thickness d20 and an inner surface 120.The thin walled silica tube 12, which comprises a conical closure 125 atits lower end, is used according to the inventive method as overcladdingtube 12. Silica tubes of this kind are available from severalmanufacturers.

FIG. 3 shows the primary preform 11 held in a centrally insertedposition within the overcladding tube 12 with said first and secondprimary axes x1, x2 in substantial alignment with each other.

The thickness d20 of the circular wall of the overcladding tube 12 isfor example ten times smaller than its inner diameter d2. However theratio of said diameter/thickness d2/d20 may be up to 50 and higher. Theratio d2/d1 of the inner diameter d2 of the overcladding tube 12 and theouter diameter d1 of the primary preform 11 is for example in the rangeof 1.5 up to 5 and more.

Hence, the volume of the interior space 15, that is defined by the outersurface 111 of the primary preform 11 and inner surface 120 of theovercladding tube 12 is relatively large, i.e., several times largerthan the volume of the primary preform 11.

FIG. 4 shows an unprocessed secondary preform 1 with the primary preform11 and the overcladding tube 12 of FIG. 3 with the interior space 15filled with overcladding grain 13, a pure or doped synthetic silicagrain or powder, that is selected according to the desired properties ofthe fiber during the drawing process or in view of its laterperformance.

FIGS. 1 a, 2 a, 3 a and 4 a show crosssections of the primary preform11, the overcladding tube 12 and the overcladding grain 13 along line sin FIGS. 1 to 4.

FIG. 5 shows the secondary preform 1 of FIG. 4 with an adjoiner 3inserted into the overcladding tube 12, holding the primary preform 11in centralized position and closing and sealing the interior space 1 onthe upper side. In this embodiment of the invention the overcladdinggrain 13 had been inserted into the interior space 15 before theadjoiner 3 has been mounted.

FIG. 6 shows the primary preform 11 and the overcladding tube 12 alignedand covered by means of an adjoiner 3, that comprises a channel 38,through which the overcladding grain 13 can be inserted.

The adjoiners 3 shown in FIGS. 5 and 6, which have a first primary axisx3, further comprise evacuation channels 32, 33 through which, by meansof a vacuum pump 22, the secondary preform 1, that is filled withovercladding grain 13, can be evacuated.

FIGS. 5 and 6 further show a heat supply or furnace 23, which allowsheating of the secondary preform 1 at its lower end for example totemperatures in the range of to 2100° C. to 2350° C. Due to the thermalenergy provided by the furnace 23 and due to the established differenceof pressures that are present in and outside the secondary preform 1,the overcladding tube 12 will collapse and press the molten overcladdinggrain 13 onto the primary preform 11. Thus, the overcladding material ofthe overcladding tube 12 and the overcladding grain 13 will form apractically homogeneous layer that adjoins the primary preform. FIGS. 5a and 6 a symbolically show a crosssection of the secondary preform 1after performing the melting process.

Fusing of the secondary preform 1 and fiber drawing can be performedsimultaneously. However it is also possible to process the secondarypreform 1 completely before the fiber is drawn.

FIG. 6 shows in a sectional view of the upper end of secondary preform 1of FIG. 5 in detail. The adjoiner 3, which is inserted into theovercladding tube 12, comprises two peripheral circular grooves withsealing elements, e.g. o-rings, that tightly adjoin and seal the innersurface 120 of the overcladding tube 12 so that the interior space 15that is limited by the adjoiner 3, the outer surface 111 of the primarypreform 11 and the inner surface 120 of the overcladding tube 12 and itsclosure 125 at the lower end can be evacuated. The evacuation can beperformed through evacuation channels 32 and 33 provided in the adjoiner3 and through a tube 220 that connects the adjoiner 3 with the vacuumpump 22. The tube 220 is connected to the adjoiner 3 by means of a valve221 that can be closed after the evacuation process has been performed.Instead, for generating a condition of reduced pressure, a gas could besupplied to a corresponding channel in the adjoiner 3, as described in[2].

The adjoiner 3 shown in FIGS. 7 to 9 further comprises, coaxiallyaligned with the first primary axis x3, a cylindrical opening 31 with adiameter d3 that corresponds to the outer diameter d1 of the primarypreform 11 and, coaxially aligned with the first primary axis x3, twocylindrical segments 35 with a diameter d4 that corresponds to the innerdiameter d2 of the overcladding tube 12. The adjoiner 3 can therefore beinserted into the cladding tube 12 so that the cylindrical segments 35adjoin the inner surface 120 of the overcladding tube 12 and the primarypreform 11 is inserted into the cylindrical opening 31 which leads to anend piece 36 that either is closed or can be closed by means of asealing cap 39.

For sealing the adjoiner towards the inner surface 120 of theovercladding tube 12, two grooves, adjoining the cylindrical segments35, are provided, in which sealing elements 91 are inserted.

FIG. 8 shows the adjoiner 3 used for the secondary preform 1 of FIG. 4and FIG. 9 shows a sectional view of the adjoiner 3 of FIG. 7, with thechannel 38 provided for the insertion of overcladding grain 13. In FIG.9 it is further shown that the first evacuation channel 32 is arrangedconcentrically to the primary axis x3 of the adjoiner 3 with a diameterd5 that is significantly larger than the diameter d3 of the adjoiningcylindrical opening 31.

FIG. 10 shows an apparatus used for drawing an optical fiber 5 from thesecondary preform 1 of FIG. 5. Once the secondary preform 1 is heated toits melting point and a fiber 5 has been pulled, an angular area calledthe neckdown is formed. A single optical fiber 5 emerges from thepreform in a semi-molten state and passes through a diameter monitor 24.The optical fiber 5 continues to be pulled downward and passes through acoating applicator 25 that applies a coating to protect the opticalfiber 5. The optical fiber 5 also passes through other units 26, 27 thatcure the optical coating and monitor the overall diameter after thecoating has been applied. The optical fiber 5 than encounters a spinningapparatus 28 which may comprise a roller that imparts a spin into theoptical fiber. The optical fiber 25 then eventually encounters a seriesof rollers (not shown) pulling the fiber before the optical fiber isthen wrapped around a drum or spool 29. The secondary preform 1 ismounted in a holding device 21, which allows controlled verticalmovement along and preferably rotation around its axis x123. Furthermorethe holding device 21 may be designed to apply a vibration onto thesecondary preform in order to condense the overcladding grain 13provided in the interior space 15.

What has been described above is merely illustrative of the applicationof the principles of the present invention. Other arrangements can beimplemented by those skilled in the art without departing from thespirit and scope of protection of the present invention. Dimensions ofthe primary preform 11 and the overcladding tube 12 can be selected in awide range as well as the granularity of the overcladding grain orpowder 13. It is important to note that the dimensions are not limitedto the examples defined above. Materials are selected according to themanufacturing parameters and properties desired for the fabricatedoptical fiber. The channels and openings 31, 32, 33, 38 and sealingmeans 34, 39, 91 for the adjoiner 3 can be designed in various ways. Theclosure 125 at the lower end of the overcladding tube 12 can have formsthat significantly differ from a conical form. However the closure 125and the lower end of the primary preform are preferably matched in orderto facilitate alignment. Conditions for drawing a fiber can be appliedand optimised in a known manner (see e.g. [5], EP 1 384 700 A1), so thatoptimal operating parameters, such as furnace temperature and drawingspeed, can be found. Hence, such operating parameters are not limited byvalues mentioned above.

REFERENCES

-   [1] Mool C. Gupta, Handbook of PHOTONICS, CRC Press, 1997 Boca    Raton, chapter 10.7, pages 445-449-   [2] U.S. Pat. No. 6,519,974 B1-   [3] U.S. Pat. No. 2,980,957-   [4] U.S. Pat. No. 4,820,322-   [5] EP 1 384 700 A1-   [6] U.S. Pat. No. 6,047,568-   [7] U.S. Pat. No. 6,334,338

1. A method for fabricating a secondary processed optical fiber preform,comprising the steps of: inserting a primary optical fiber preformhaving an outer surface into a thin-walled, elongated overcladding tubehaving an inner surface, wherein the outer surface and the inner surfacedefine an interior space limited at a first end of the thin-walled,elongated overcladding tube by a conically formed closure; holding theprimary optical fiber preform in a substantially longitudinally coaxialrelationship with the thin-walled, elongated overcladding tube;supplying solely overcladding grain into the interior space, wherein theovercladding grain is a homogeneous crystalline silica grain that isselected from the group consisting of pure or doped synthetic silicapowder; forming an unprocessed secondary preform, wherein saidunprocessed secondary preform comprises; the primary optical fiberpreform, the thin-walled, elongated overcladding tube, the conicallyformed closure, and overcladding grain, limiting the interior space at asecond end of the overcladding tube by an adjoiner; generating a reducedpressure within the interior space; and heating at least a portion ofthe unprocessed secondary preform in order to transform at least theheated portion to a processed secondary preform from which an opticalfiber can be drawn.
 2. The method according to claim 1, wherein theinner diameter of the thin-walled, elongated overcladding tube is atleast 1.5 times larger than the outer diameter of the primary opticalfiber preform and more than 10 times larger than the wall thickness ofthe thin-walled, elongated overcladding tube.
 3. The method according toclaim 1, wherein the overcladding grain, which consists of particleswith a smaller diameter, is inserted after the adjoiner is mounted,through a channel provided therein.
 4. The method according to claim 1,wherein the temperature for heating at least a portion of theunprocessed secondary preform, is selected in the range of 2100° C. to2350° C.
 5. The method according to claim 1, wherein the unprocessedsecondary preform is heated over its entire length in order to obtain aprocessed secondary preform.
 6. The method according to claim 1, furthercomprising sealing the interior space with the adjoiner, the adjoinerhaving a first hollow cylindrical portion and a second hollowcylindrical portion, the first hollow cylindrical portion and secondhollow cylindrical portion both having an outer diameter thatcorresponds to an inner diameter of the overcladding tube, the firstcylindrical portion having an inner diameter that is larger than aninner diameter of the second cylindrical portion.
 7. A secondary preformfor fabricating an optical fiber comprising; an elongated primaryoptical preform with an outer surface and an elongated overcladding tubewherein the primary optical preform is disposed coaxially within theelongated overcladding tube, said elongated overcladding tube having aninner surface and a conical closure at a first end thereof, wherein theconical closure, the outer surface, and the inner surface define aninterior space that is filled solely with overcladding grain, whereinthe overcladding grain is a homogeneous crystalline silica grain that isselected from the group consisting of pure or doped synthetic silicapowder.
 8. A secondary preform according to claim 7, wherein theelongated primary optical preform and the elongated overcladding tubeare held and sealed at a second end of the elongated overcladding tubeby an adjoiner comprising: an opening coaxially aligned with the primaryoptical preform, the opening having a diameter to generally fit theouter diameter of the elongated primary optical preform, at least onecylindrical element coaxially aligned with the elongated primary opticalperform, and a sealing element with a diameter to generally fit thediameter of the elongated overcladding tube, and an evacuation channelthat is connectable to a vacuum pump or to a gas supply.
 9. A secondarypreform according to claim 8, wherein the adjoiner further comprises atleast one channel for inserting the overcladding grain into the interiorspace.
 10. A secondary preform according to claim 7, wherein theovercladding grain is molten.
 11. The secondary preform according toclaim 7, further comprising an adjoiner that seals the elongatedovercladding tube, the adjoiner having a first hollow cylindricalportion and a second hollow cylindrical portion, the first hollowcylindrical portion and second hollow cylindrical portion both having anouter diameter that corresponds to an inner diameter of the overcladdingtube, the first cylindrical portion having an inner diameter that islarger than an inner diameter of the second cylindrical portion.
 12. Amethod for fabricating an optical fiber, comprising the steps of:inserting a primary optical fiber preform having an outer surface into athin-walled, elongated overcladding tube having an inner surface,wherein the outer surface and the inner surface define an interior spacelimited at a first end of the overcladding tube by a conically formedclosure; holding the primary optical fiber preform in a substantiallylongitudinally coaxial relationship with the elongated overcladdingtube; supplying solely overcladding grain into the interior space,wherein the overcladding grain is a homogeneous crystalline silica grainthat is selected from the group consisting of pure or doped syntheticsilica powder; forming an unprocessed secondary preform, wherein saidunprocessed secondary preform comprises; the primary optical fiberpreform, the thin-walled, elongated overcladding tube, the conicallyformed closure, and overcladding grain, limiting the interior space atthe second end of the elongated overcladding tube by an adjoiner;generating a reduced pressure within the interior space; heating atleast the bottom portion of the unprocessed secondary preform; and,drawing an optical fiber from a bottom portion of the unprocessedsecondary preform simultaneously with the step of heating.
 13. Themethod according to claim 12, wherein the inner diameter of theelongated overcladding tube is at least 1.5 times larger than the outerdiameter of the primary optical fiber preform and more than 10 timeslarger than the wall thickness of the elongated overcladding tube. 14.The method according to claim 12, wherein the overcladding grain isinserted after the adjoiner is mounted, through a channel providedtherein.
 15. The method according to claim 12, wherein the temperaturefor heating at least the bottom portion of the unprocessed secondarypreform, is selected in the range of 2100° C. to 2350° C.
 16. The methodaccording to claim 12, further comprising sealing the interior spacewith the adjoiner, the adjoiner having a first hollow cylindricalportion and a second hollow cylindrical portion, the first hollowcylindrical portion and second hollow cylindrical portion both having anouter diameter that corresponds to an inner diameter of the overcladdingtube, the first cylindrical portion having an inner diameter that islarger than an inner diameter of the second cylindrical portion.